Designs and variants
SKF motor encoder units are compact, integrated units consisting of (fig. 1
an SKF Explorer deep groove ball bearing in the 62 series with a snap ring groove in the outer ring (→ Single row deep groove ball bearings) and an RS1 contact seal
an impulse ring
a sensor body
a connecting cable
The impulse ring, which attaches to the inner ring of the bearing, is a composite magnetized ring that contains between 32 and 80 north and south poles. The number of poles depends on the size of the bearing. The sensor body, which is attached to the outer ring, protects the patented SKF Hall effect cell. The multi-wire connecting cable extends in the radial direction.
The bearing is protected by a contact seal on one side. On the opposite side of the bearing, the impulse ring and sensor body create an effective labyrinth seal to keep lubricant in and solid contaminants out of the bearing.
SKF motor encoder units use a compact and robust sensor that produces an incremental encoder signal. The sensor is accurate down to zero revolutions per minute. An integrated active circuit (requiring an external voltage supply) in the sensor body contains two Hall effect cells that produce an output signal consisting of two square waves (fig. 2
The signals can be interpreted by motor controllers in different ways:
The direction of rotation can be determined from the phase shift, when the rising edge of a signal first appears.
Low speeds can be determined by measuring the time between two electrical events, such events being the rising and falling edge on either square wave.
High speeds can be measured by counting the number of electrical events within a given time period.
The two square waves are 90° out of phase with each other. This phase shift changes sign with the direction of rotation. Fig. 2 shows the general specifications of the signal. The presence of two signals in quadrature enables a processing unit to multiply the number of angular position increments per revolution. For example, using a standard SKF sensor bearing with 64 pulses per revolution and a standard electronic interface that can detect the rising (Low/High) and falling (High/Low) times of each of the two signals, it is possible to obtain 256 electrical events per revolution, which translates to an angular resolution of 1,4° (fig. 2).
SKF motor encoder units provide accurate and reliable signals for effective motor control and are 100% tested for period accuracy, duty cycle and phase shift during manufacture.
SKF motor encoder units are available as standard with one of the following:
a free cable end with an output signal consisting of two square waves, designation suffix 008A (fig. 3)
an AMP Superseal™ connector (AMP Nos. 282106-1 and 282404-1), designation suffix 108A (fig. 4)
Standard cable lengths are listed in the product table
. For alternative connectors or cable lengths, contact SKF.
SKF motor encoder units are:
filled, under clean conditions, with a high-quality grease (table 1) that is suitable for the most common operating conditions of electric motors
The grease life in the bearing can be calculated according to the method described under Grease life for capped bearings.
Motor encoder units for extreme operating conditions
Magnetic sensors have temperature and motor power limits. For applications where magnetic sensors are not practical, high-performance inductive technology can be used instead. Inductive sensors use coils to sense the rotation of a specially designed induction tooth ring.
For additional information about motor encoder units for extreme operating conditions, contact SKF.